Electrophotographic image formation comprises developing an electrostatic latent image formed on a photoreceptor with a toner comprising a colorant dispersed in a binder resin, transferring the toner image to receiving paper, and fixing the transferred toner image by means of, for example, a hot roll. The photoreceptor is cleaned after the transferring step to prepare for next latent image formation.
Metal oxide particles are often combined with toner particles in order to improve selected properties of the toner particles, including fluidity, transferability, fixability, and cleaning properties. Typically, the metal oxide particles, e.g., silica, alumina, or titania, are subjected to a chemical treatment to render the surface of the metal oxide particles hydrophobic.
In addition, the metal oxide particles strongly influence the chargeability, i.e., tribocharge, of the toner composition. Specifically, toner containing silica as an additive exhibits higher absolute levels of tribocharge than toner containing titania, and toner containing titania exhibits higher absolute levels of tribocharge than toner containing alumina.
However, it is known that the tribocharge of toner containing silica depends on the temperature and relative humidity of the environment. Often, the tribocharging of toner formulated with silica is too high at low humidity and too low at high humidity. As a result, silica produces a large difference in chargeability depending on the environmental conditions, which leads to impaired transferability and ultimately reduced image quality. In contrast, the tribocharging of toner formulated with aluminum oxide is often too low under both high and low humidity conditions. While the chemical treatment to render the surface of the metal oxide particles hydrophobic also affects the tribocharge of toner containing silica and aluminum oxide, the chemical treatment generally fails to overcome the deficiencies of the tribocharge properties of the core metal oxide particle.
Thus, a need exists for toner compositions with improved humidity resistance and/or levels of tribocharge. Specifically, a need exists for chemical treatments of metal oxide particles that allow for control of the tribocharge properties and humidity resistance of the metal oxide particle additives in toner.